GOP Sen. Lamar Alexander won't seek reelection

Sen. Lamar AlexanderAndrew (Lamar) Lamar AlexanderHere are the lawmakers who aren't seeking reelection in 2020 EXCLUSIVE: Swing-state voters oppose 'surprise' medical bill legislation, Trump pollster warns The 13 Republicans needed to pass gun-control legislation MORE (R-Tenn.) on Monday announced that he will not run for reelection in 2020.

"I will not be a candidate for re-election to the United States Senate in 2020. The people of Tennessee have been very generous, electing me to serve more combined years as Governor and Senator than anyone else from our state," Alexander said.

Alexander, who has been in the Senate since 2003, said he will serve out the remainder of his term, which runs through the end of 2020. 

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"I have gotten up every day thinking that I could help make our state and country a little better, and gone to bed most nights thinking that I have. I will continue to serve with that same spirit during the remaining two years of my term," he added.

Alexander, 78, is the first senator up in 2020 to announce that they won't seek reelection. Republicans face a challenging map during the next cycle, where they'll be defending approximately 22 seats, including in states like Colorado and Maine, which Democratic presidential nominee Hillary ClintonHillary Diane Rodham ClintonGOP struggles with retirement wave Overnight Energy: Trump to revoke California's tailpipe waiver | Democrats propose bill to revoke Trump endangered species rollback | Trump officials finalize rule allowing fewer inspectors at pork plants Mark Mellman: The most important moment in history? MORE won in 2016.

Alexander is also the latest in a string of GOP chairmen from the establishment wing of the party to announce that they will retire during the Trump era: Senate Finance Committee Chairman Orrin HatchOrrin Grant HatchTrump to award racing legend Roger Penske with Presidential Medal of Freedom Trump awards Presidential Medal of Freedom to economist, former Reagan adviser Arthur Laffer Second ex-Senate staffer charged in aiding doxxing of GOP senators MORE (R-Utah) and Senate Foreign Relations Committee Chairman Bob CorkerRobert (Bob) Phillips CorkerTrump announces, endorses ambassador to Japan's Tennessee Senate bid Meet the key Senate player in GOP fight over Saudi Arabia Trump says he's 'very happy' some GOP senators have 'gone on to greener pastures' MORE (R-Tenn.) are retiring at the end of the current Congress.

Corker, on Monday, called working with Alexander "one of the highlights" of his work in the Senate.

"As one of the finest statesmen our state has ever seen, Lamar will leave behind a remarkable legacy. I know he will press through the next two years with great vigor, and I look forward to all he will accomplish on behalf of Tennesseans as he completes his service in Washington," Corker said in a statement.

Alexander previously served as chairman of the Senate Republican Conference and is considered to be close to both Majority Leader Mitch McConnellAddison (Mitch) Mitchell McConnellDemocrats seize Senate floor to protest gun inaction: 'Put up or shut up' Democrats press for action on election security Hillicon Valley: Election security looms over funding talks | Antitrust enforcers in turf war | Facebook details new oversight board | Apple fights EU tax bill MORE (R-Ky.) and Minority Leader Charles SchumerCharles (Chuck) Ellis SchumerSchumer, Pelosi push Trump to back universal background check bill Sinema says she would back Kennedy in race against Markey Democrats threaten to withhold defense votes over wall MORE (D-N.Y.).

He's also been at the center of some of the biggest fights in the Trump administration because of his position as chairman of the Health, Education, Labor and Pensions (HELP) Committee. 

He was part of a working group of GOP senators, assembled by McConnell, that crafted the Senate GOP's failed ObamaCare replacement bill last year. 

But Democrats have credited him with being willing to work with them on key issues despite growing partisanship in the Senate. 

He crafted a deal with Sen. Patty MurrayPatricia (Patty) Lynn MurrayEXCLUSIVE: Swing-state voters oppose 'surprise' medical bill legislation, Trump pollster warns Overnight Health Care: Juul's lobbying efforts fall short as Trump moves to ban flavored e-cigarettes | Facebook removes fact check from anti-abortion video after criticism | Poll: Most Democrats want presidential candidate who would build on ObamaCare Trump's sinking polls embolden Democrats to play hardball MORE (D-Wash.), the top Democrat on the health committee, to funds key payments to health insurers for two years.

He was also one of less than 10 GOP senators who voted for a bipartisan immigration proposal earlier this year that would have provided approximately 1.8 million immigrants brought to the United States illegally as children an eventual pathway to citizenship in exchange for $25 billion in border security. 

Sen. Ed MarkeyEdward (Ed) John MarkeyGreta Thunberg scolds Congress on climate action: 'I know you are trying but just not hard enough' Obama meets with Greta Thunberg: 'One of our planet's greatest advocates' Young insurgents aren't rushing to Kennedy's side in Markey fight MORE (D-Mass.) on Monday also credit Alexander as being a "true partner" in addressing the country's opioid epidemic. 

Alexander's decision to retire will leave a hole in Tennessee politics, where he has been a mainstay for decades. He was first elected to the Senate in 2002, previously served as governor of Tennessee and made two unsuccessful bids for the White House in 1996 and 2000.

But he was considered potentially vulnerable to a primary challenge in 2020 after winning narrowly fending off tea party challenger Rep. Joe Carr during the 2014 GOP primary. 

It's unclear who will vie to succeed Alexander. Tennessee Gov. Bill Haslam (R) flirted with a Senate bid in 2018. And state Sen. Mark GreenMark GreenInterior gains new watchdog We need a new structure to secure our border Tackling China in modern Cold War MORE (R) hinted last year that he could make a Senate run in 2020.  

Green told USA Today in August that he would be "back in the election cycle" in 2020.

"I think that's going to be an opportunity for me potentially later," he added about a potential Senate run. 

President TrumpDonald John TrumpJimmy Carter: 'I hope there's an age limit' on presidency White House fires DHS general counsel: report Trump to cap California trip with visit to the border MORE won Tennessee by more than 26 points in 2016. In last month's election to replace Corker, Rep. Marsha BlackburnMarsha BlackburnThe evolution of Taylor Swift's political activism Kellyanne Conway responds to Taylor Swift criticism by invoking pop star's lyrics Is there internet life after thirty? MORE (R-Tenn.) defeated former Gov. Phil Bredesen (D) by nearly 11 points.

Updated at 1:12 p.m.